1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly, an original transporting and reading apparatus, which is used, for example, in a copying apparatus, and which includes a device for transporting an original sheet to an image reading position at a transparent plate where a light exposure type image reading operation is performed.
2. Description of the Related Art
As one of various kinds of image reading apparatuses, there is an image reading apparatus, which includes a device for automatically transporting an original sheet to an image reading position of the image reading apparatus.
Compared with other types of the image reading apparatuses to which the original sheet is manually feeded or placed, the image reading apparatus with such an automatic transporting device is advantageous in its easy and speedy operation.
Such a kind of transporting device is provided on the upper side of a main body of the image reading apparatus and includes a plurality of rollers and paper guides to transport the original sheet from an original hopper thereof to the image reading position.
The main body includes the transparent plate at one portion of its upper surface, which is made of glass etc. This transparent plate offers the image reading position of the image reading apparatus.
The transporting device includes a pressure roller for pressing the original sheet onto the transparent plate at the pressure applying position. The pressure roller is made of material of high friction coefficient at its surface so as to effectively transport the original sheet through the transparent plate. Thus, a good contact between the original sheet and the transparent plate is achieved at this pressure applying position during the transportion of the original sheet along the transparent plate.
In case that the image reading apparatus is used in the copying apparatus, the image reading apparatus, for example, performs a slit-exposure process by illuminating a light onto the original sheet through the transparent plate, and by detecting a slitted light out of the reflection light from the original sheet through the transparent plate and a slit. The slitted light is guided onto the photo-sensitive drum so as to form an latent image.
Thus, at each slit-exposure process, a slitted recutangular image of the original sheet, which has a predetermined width and length, is formed on the photo sentitive drum, in such a manner that the longitudinal axis of the slitted light is positioned on the pressure applying position and parallel to the wide direction of the original sheet. This slit-exposure process is repeated as the original sheet is transported, resulting in forming all the image of the original sheet onto the photo-sensitive drum.
Then, a toner image is formed on the photo-sensitive drum, and the toner image is transferred onto the recording sheet to output the image copied recording sheet.
In the above mentioned transporting device, taking a view of spatial arrangement, the original sheet is transported to the transparent plate from the upper side off to the right or left of the pressure roller, then illuminated by the light on the transparent plate at the pressure applying position of the pressure roller, and then transported toward the upper side off to the left or right of the pressure roller, back to the original hopper.
In the above mentioned transporting device, when the original sheet is transported from the pressure roller to the next transporting roller which is positioned above the pressure roller off to the left or right, the original sheet is pushed from the behind by the pressure roller and transported obliquely and upwardly into a curved transporting path. Thus, because of the stiffness and elasticity of the original sheet, the original sheet gradually separates from the surface of the transparent plate right after passing through the pressure applying position of the pressure roller.
In other word, at a downstream side, with respect to the transporting direction, of the pressure applying position, the original sheet does not have a good contact with the transparent plate, instead, an undesirable phenomenon called "floating" of the original sheet occurs due to its stiffness and elasticity.
On the contrary, at the upstream side of the pressure roller, the original sheet is transported to the pressure roller by the previous transporting roller which is positioned above the pressure roller off to the right or left, so that the original sheet is pushed from the behind by this transporting roller and transported obliquely and downwardly from a curved transporting path. Thus, the original sheet presses itself onto the transparent plate by its rigidity and elasticity in the vicinity of the pressure roller at this upstream side.
Accordingly as described above, the original sheet has a good contact with the transparent plate at the upstream side of the pressure applying position for a ceratin length. However, if the rigidity of the original sheet is very high, for example in a case of a very thick paper is used as the original sheet, this contacting length may become very short.
Consequently, in the above mentioned image reading apparatus in which the center of the slitted light having a certain slit width along the transporting direction is set on the pressure applying position, all portion of the slitted light can not be well focused, since the optical distance to the photo-sensitive drum from one portion of the illuminated original sheet contacting the transparent plate is different from that of other floating portion of the illuminated original sheet. Namely, about a half portion of the illuminated original sheet at the downstream side of the pressure applying position can not be image-read with a well focused condition.
Thus, there is a first problem that this off-focus degrades the quality of the image obtained by the image reading apparatus.
By the way, the above mentioned kind of image reading apparatus may suffer from the phenomenon called "off-center" of the original sheet, which means the original sheet is not properly positioned in the center of the transporting path at the pressure roller. When this off-center occurs, the dark shadow image would be formed in the edge portion of the recording sheet without any countermeasure.
Accordingly, a blanking lamp is equipped for blanking this shadow image, with the drawback that a certain amount of the image loss with respect to the edge portion of the original sheet is innevitablly caused, which is the second problem.
In the above mentioned image reading apparatus, various sizes of the original sheet can be transported. Thus, all the rollers including the pressure roller have larger width than the prescribed maximum width of the original sheet to be transported.
So, when an original sheet which has a smaller width than this maximum width is transported to the transparent plate, the pressure roller gets a direct contact with the transparent plate at one or both edges of the original sheet. Here, since the pressure roller has the high friction coefficient as aforementioned, when this pressure roller rotates with such a condition of contacting the transparent plate, the driving resistance is greatly increased.
Accordingly, a large torque is required to rotate the pressure roller, where a large proportion of the driving force for the pressure roller is consumed against this frictional force, with a drawback of making it very difficult to transport the original sheet smoothly. Namely, in this configuration, each time the pressure roller is slipped on the transparent plate, the transporting speed of the original sheet is varied, making the movement of the original sheet clumsy, though it is supposed to be constant to perform a proper slit-exposure process.
Consequently, the expansion and shrinkage of the image obtained by the image reading apparatus is caused in the direction corresponding to the transporting direction of the original sheet, which is the third problem.